The Celestial Festival is a big festival where Orem Vacker uses his skills as a Flasher to make beautiful light formations in the sky. It is a big deal in the Elvin world, and one of their most celebrated occasions.
In Book 2: Exile, Sophie teleported in on Silveny's back at the end and circled the crowd. This addition was to help calm the Elvin people's anxiety over Sophie and Dex's kidnapping.